The Great Wall of China, winding 1,500 miles over mountain and plain, and from 15 to 50 feet high and 15 to 25 feet wide, is a reminder of the country's long struggle to keep out invaders from the North. For centuries it served as a boundary line between China and Mongolia. To reach the Krimmer Mennonite Brethren mission field in Inner Mongolia, missionaries had to cross the Great Wall. On the other side, shadowed by this barrier, they began the task of "breaking down walls" to the message of the Christian gospel. In this book, A.K. and Gertrude Wiens describe the years of Christian mission activity in Inner Mongolia before the "doors were closed" in the 1940s.